Outer space is the expanse that exists beyond Earth’s atmosphere but a clear boundary between the two is impossible to define because the density of atmosphere gradually decreases as altitude increases.
However, the Kármán line at 100 km above sea level is generally used to mark the beginning of outer space in international space treaties and is accepted by the Fédération Aéronautique Internationale as the boundary between aeronautics and astronautics.
The Kármán line is based on the work of physicist Theodore von Kármán who determined that the atmosphere at an altitude of 100 km becomes too thin to support aeronautical flight because it would have to travel faster than orbital velocity to generate aerodynamic lift.
Beyond the Kármán line, 107 countries are bound by the Outer Space Treaty which provides a basic legal framework for international space law. In particular, it precludes claims of national sovereignty, promotes the free exploration of space, and bans the placement of weapons from Earth’s orbit.